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  • Let's talk about rainstorm warning (Part 2): Q&A

  • Tuesday, 10th April 2018

Last time, we discussed the purpose of and the factors being considered in establishing the rainstorm warning system. We will look at some commonly asked questions this time.

Q: Why not implement district-level rainstorm warning?

The development and movement of a rainstorm are very quick at times and can change greatly in a short time. If district-level rainstorm warning were implemented, the warning levels for different districts at different time would likely be rather different, causing confusion to those who commute cross different districts. This will also pose great challenges to the media in conveying the rapidly changing warning messages to the public.

Taking the heavy rain episode on the morning of 25 May 2013 as an example (Figure 1), the hourly rainfall over Lantau Island, Lamma Island and Wong Chuk Hang once exceeded 40 millimetres (yellow region in Figure 1). The rain band subsequently moved northwards quickly. The rain over those districts previously under downpour subsided swiftly, while that over Tsing Yi, Tsuen Wan and Ma On Shan became heavy. If district-level rainstorm warning were implemented, the warning levels for the districts mentioned above would change within a short time, and the public would find it difficult to cope with: should they continue to go to work or to school? Should their decision be made based on the warning level of the departing district or that of destination? Should the district enroute to work or to school be also taken into consideration? The situation will then become highly complicated.

Figure 1

Figure 1 Rainfall distribution at 10:45 a.m. and 11:30 a.m. on 25 May 2013.

Q: Why were there occasions when the rain was not heavy or there was even no rain at my location while the Observatory had issued the rainstorm warning?

There are some degrees of randomness in where heavy rain falls and how its intensity changes. Taking the "Red Rainstorm" on the morning of 10 May 2016 as an example (Figure 2), most parts of the New Territories were affected by the torrential rain, yet rain was not heavy over Lantau Island. There was even no rain in many places of Hong Kong Island and Kowloon. To get hold of the changes in rainstorm, one can refer to the Observatory's radar imageries (Figure 3 - red, orange and yellow mean heavier rainfall) for the location, intensity and direction of movement of the rain area.

Figure 2

Figure 2 Distribution of past-hour rainfall at 7:30 a.m. on 10 May 2016.

Figure 3

Figure 3 Radar imagery at 7:30 a.m. on 10 May 2016.

Q: Why can't the Observatory predict the arrival of rainstorm a couple of hours in advance so as to avoid issuing the Red or Black Rainstorm Warning during "embarrassing time" (e.g. the time when travelling to work or to school) and minimize inconvenience?

Rainstorm can change vastly in a matter of an hour or even less. Issuing the Red or Black Rainstorm Warning accurately or providing early alert a couple of hours in advance is still a challenge at the current level of science and technology. If we often issue the rainstorm warning too early such that the rain band suddenly weakens or moves elsewhere, the adverse effect of "crying wolf" will come sooner or later. Taking the situation on the night of 6 May 2016 as an example (Figure 4), there was a very broad area of intense rain approaching Hong Kong from the northwest, but it weakened abruptly right before reaching Hong Kong and then skirted the north of the territory. This exemplifies the difficulty of location-based and quantitative forecast of rain. It is difficult to issue rainstorm warning a couple of hours in advance. If a warning has to be issued at the so-called "embarrassing time", while it will inevitably cause inconvenience to people going to work or to school, we hope the public can understand that such decision is made for the sake of safety. We advise every sector in the society to formulate contingency arrangements for rainstorm prior to the rain season so that they can be readily deployed when necessary.

Figure 4

Figure 4 Radar imageries at 7:30 p.m. (left) and 9:30 p.m. (right) on 6 May 2016.

L.S. Lee      M.C. Chow

  • Let's talk about rainstorm warning (Part 1): Origin

  • Tuesday, 3rd April 2018

Origin of rainstorm warning

On the morning of 8 May 1992, the Observatory recorded some 110 millimetres of rainfall in an hour that set the highest hourly rainfall record at that time [Note 1]. There were serious flooding and landslips in places on that day while the traffic in various districts was paralysed. Tragically, even some people lost their lives. This rainstorm led to the establishment of the Rainstorm Warning Signal, which aims to reflect objectively the impact of heavy rain on our society. It also aims to caution the public about the major disruption that a rainstorm may bring, so as to enable them to take precautionary measures to avoid loss of life and property.

Time flies. The rainstorm warning system has been implemented for over a quarter-century. During 2013 to 2017, the Observatory had issued the Black Rainstorm Warnings five years in a row. As extreme weather will become more frequent due to global warming, a better understanding of the origin and meaning of the rainstorm warning system will enable us to respond timely under adverse weather.

Considerations of rainstorm warning

Although Hong Kong is small in area, its terrain is complex and the impact of heavy rain, flooding or landslip in different regions is not the same. The rainstorm warning should be operated with due consideration of the overall emergency response. We cannot consider solely the spatial coverage of heavy rain, but also need to take into consideration such risk factors as heavy-rain-induced flooding and landslip as well as the size of the affected population. Currently, data of over 100 rain gauges scattered around Hong Kong is being used by the Observatory to determine whether or not to issue a rainstorm warning signal and, if issued, the level of warning. These rain gauges are not distributed evenly across the territory. There are a number of factors being considered. The rain gauges are denser in areas of higher risk of flood and landslip and higher population density (Figure 1). If heavy rain falls over areas of denser rain gauges, the number of rain gauges recording high rainfall level will then be higher, and thereby higher chance of issuing rainstorm warning. In the event that the amount of rainfall reaches a certain level that the risk of landslip in Hong Kong becomes high and numerous landslips are likely, the Observatory will also issue a landslip warning [Note 2].

Figure 1

Figure 1
(a)  Distribution of severe flooding cases from 2007 to 2016 (based on data provided by the Drainage Services Department). The blue line indicates the area applicable to the Special Announcement on Flooding in the northern New Territories;

(b)  Distribution of landslip locations from 2007 to 2016. The redder the colour, the higher the number of landslip cases in the area. Red colour represents over 10 landslip cases per square kilometre in general (based on data provided by the Civil Engineering and Development Department);

(c)  Population density in 2016 according to District Council districts (based on data provided by the Census and Statistics Department);

(d)  Density distribution of reference rain gauges. Darker colour indicates a higher density of rain gauges, while lighter colour represents a lower density. Dark green means a density of over 3 rain gauges in 10 square kilometres in general.

Taking the heavy rain episode on the night of 8 June 2014 as an example (Figure 2), the Observatory issued the Amber and the Red Rainstorm Warning Signals in succession that night. While the rain was heavier on Hong Kong Island, Kowloon and Tseung Kwan O, there was generally not much rain over other parts of the New Territories. No rain was even recorded in some places. This episode of rain happened to fall over the areas of higher risk of impact brought by weather and of higher population density. The Observatory eventually issued the Red Rainstorm Warning and serious flooding affected many roads on Hong Kong Island and in Kowloon.

Figure 2

Figure 2 Rainfall distribution at 10:15 p.m. on 8 June 2014.

As for another heavy rain episode that affected mainly the northern part of the New Territories on the morning of 24 July 2015 (Figure 3), the Observatory did not issue a rainstorm warning, but issued the Special Announcement on Flooding in the northern New Territories that was tailored for that area. The rainfall distribution map shows that the hourly rainfall in most parts of Hong Kong did not reach 30 millimetres, except for the northern part of the New Territories. Hence, the rainstorm warning was not issued. To conclude, the overall impact of this heavy rain episode on Hong Kong was relatively low.

Figure 3

Figure 3 Rainfall distribution at 8:45 a.m. on 24 July 2015.

Localised heavy rain

Given that rainstorm warning is a territory-wide warning system directly affecting all people going to work and to school rather than a system catering for localised situation, the Observatory launched the Announcement on Localised Heavy Rain service in the summer of 2016. When rain is particularly heavy in individual districts of Hong Kong, and yet to reach the criteria of the Red or Black Rainstorm Warning Signal, the Observatory will indicate the affected districts and the recorded rainfall in the Announcement on Localised Heavy Rain to remind the public to take precautions against the risk of flooding. Since flooding may lag behind the rain, the risk will not abate immediately even when the rain starts to ease off.

"Flooding can happen even it is just Amber"

It is worth noting that when the Amber Rainstorm Warning is in force, flooding may occur in some low-lying or poorly drained areas, and rivers may also overflow. Therefore, the impact of the Amber Rainstorm Warning should never be underestimated. The Red and the Black Rainstorm Warnings indicate even worse situations. In particular, the Black Rainstorm Warning indicates an adverse situation with a high chance of serious flooding, river overflow, landslip, and traffic paralysis. The public should stay in safe places to avoid any danger.

Threshold of warning

Like the tropical cyclone warning signal, the rainstorm warning signal is a territory-wide warning system. However, the uncertainty of rainstorm is much greater than that of tropical cyclone. Heavy rain can develop in a short time, and the intensity of rain and the movement of rain band have some degrees of randomness. It is still a challenge to precisely predict them with the current science and technology.

One may wonder, what about lowering the threshold for the rainstorm warning, for example, issuing rainstorm warning as long as heavy rain occurs in certain individual areas (even though rain is not heavy in all other areas), or lowering the rainfall criteria for all levels of rainstorm warning (such as lowering the criterion of the Amber Rainstorm Warning from 30 millimetres per hour to 20 millimetres per hour, or even less)? If that is the case, the following situation may occur: the number of rainstorm warnings issued will increase substantially, yet the overall impact of rainfall on Hong Kong may not be serious every time. Over time, the public may feel accustomed to the warning, or even feel annoyed, eventually lowering their alertness to the rainstorm warning or even becoming negligent. When a destructive rainstorm comes then, the public may be caught off guard, causing unwanted loss of property or even human life.

Safeguarding public safety through science

The Observatory has always been serving the public with science and safeguarding public safety as its primary mandate. Through various weather warnings and information, the Observatory strives to remind the general public of the risks they may face. The Observatory also constantly performs reviews and seeks improvement, such as examining the distribution of the reference rain gauges of the rainstorm warning system, and providing more detailed weather information at the location of users through the MyObservatory mobile application. We advise members of the public not only to pay attention to the Observatory's information, but also assess the risks they may face based on their own experience and current situation. When necessary, they should stay away from dangerous places and seek assistance from emergency departments immediately.

Next time, we will talk about topics on rainstorm warning signal that you may be interested, such as the feasibility of implementing a district-level rainstorm warning, or the possibility of providing alert a couple of hours in advance before the rainstorm occurs, similar to that for tropical cyclone warning.

L.S. Lee      M.C. Chow

Note 1: New records were made subsequently in 2006 and 2008.
Note 2: The rainstorm warning aims to alert the public to the impact brought by rainstorms, whereas the landslip warning aims to alert the public to potential landslip hazards and is issued by the Observatory in consultation with the Geotechnical Engineering Office.

  • A 'shocking' Lunar New Year one hundred years ago

  • Tuesday, 20th March 2018

Shortly before the end of the lunar year in 2016 and 2018, two earthquakes brought severe damages and casualties to Taiwan and were felt in Hong Kong. However, the earthquake which caused the most severe shaking in Hong Kong in the Lunar New Year time was a magnitude 7.3 earthquake which occurred at 2:07 pm on the third day of the Lunar New Year one hundred years ago (13 February 1918). Its epicenter was located some 300 kilometres from Hong Kong near Nan'ao Island offshore of Shantou (Figure 1). It was the strongest tremor in Hong Kong in the past one hundred years or so, with an intensity of VI to VII on the Modified Mercalli Scale[1]. A number of aftershocks occurred in the two days afterwards. According to the Hong Kong Telegraph on 16 February, the most intense aftershock occurred shortly after 4 a.m. on 14 February. It was indeed a rather 'shocking' Lunar New Year for Hong Kong people in 1918.

Figure 1

Figure 1. Epicentre (red dot) and intensity distribution of the Nan'ao earthquake

(based on information from "廣東和香港地震風險概論" by Ding Yuan-zhang, 2004, Commercial Press)

The earthquake brought severe damages and casualties to Nan'ao and Shantou. The China Mail of 18 February 1918 reported that practically every house in the native quarter of Shantou was demolished and the residences and buildings of foreigners were also severely damaged. A theatre full of audiences collapsed and not a single one escaped, including the actors. Big fissures appeared on the ground in many places, with hot water and steam coming out through many of these crevices. The earthquake also damaged some historical buildings - the Longmen Pagoda on a small island north of Nan'ao tilted with the top collapsed (Figure 2), and the top of the Memorial Arch in memory of Zheng Zhi-long (father of Zheng Cheng-gong) at Shen Ao of Nan'ao also collapsed. Furthermore, the Harbour Department of Hong Kong notified that the Lammocks Lighthouse near Shantou was also damaged by the earthquake.

Figure 2

Figure 2. The Longmen Pagoda with the top damaged: one of the landmarks recording the Nan'ao earthquake (Source: Shun Chi-ming)

While the question whether this earthquake had induced widespread tsunami is still subject to further study, the following evidence suggests that it might have induced local tsunami due to underwater landslide so triggered: (a) the book 'Summary of historical natural disasters in Shantou and Chaozhou' recorded that during the earthquake, the ground shook and cracked with sea water roaring in Shantou, while tides appeared like rice soup in white at Fengshun; (b) the China Mail of 15 February reported that a ship 30 miles off Xiamen and sailing in 15 fathoms of water felt the shock distinctly, while another ship berthing at a pier in Shantou even touched the bottom, and (c) the archive of clergyman Hugh Anderson Moran had photos from an earthquake in China, which are believed to be associated with the Nan'ao earthquake[2], revealed many dead bodies lying on the shore (Figure 3).

Figure 3

Figure 3. Photos from the archive of clergyman Hugh Anderson Moran on an earthquake in China (Source: Shun Chi-ming)

In Hong Kong, 14 locally felt earth tremors occurred on 13 and 14 February 1918 according to the records of the Hong Kong Observatory (Table 1)[3]. Among them, the one occurring at 2:07 pm on 13 February was the strongest with longest duration whereas the one occurring at 4:26 am on 14 February ranked the second, consistent with the two strongest quakes at Nan'ao during the period. According to Hong Kong Telegraph of 13 February, the shock was felt all over Hong Kong, and almost all the buildings were shaking. In particular, the earthquake threw the whole Central District into a state of panic. Hundreds of people rushed out of the shaking buildings and some women and children passed out in fright. The China Mail of 14 February reported that at Yaumati, a little girl was knocked down when people rushed out of their houses in terror, receiving injuries from which she died later.

The shock at 4:26 am on 14 February was also rather intense. According to Hong Kong Telegraph of 14 February, many citizens were awakened by the tremor. The Chinese Mail reported that several people hurt their legs when falling down from their double-deck beds, and morning tea diners fled from restaurants without paying their bills.

A number of buildings in Hong Kong were damaged by the tremors in these two days. Several cracks appeared in the walls of the upstairs rooms at the Hong Kong Observatory Headquarters[3]. Most affected buildings were at the Central and Western District, including 21 Staunton Street (wall in a dangerous condition[4]), 17 Old Bailey Street (cracking from top to bottom[5], wall in a dangerous state[4]), 15 and 17 Elgin Street (cracks appeared in walls[5] and in a dangerous condition[4]), 25 Hollywood Road (walls in a dangerous condition[4]), Central Police Station (cracks appeared in walls[5]) and Hong Kong Club (cracks appeared in walls[4], some plaster fell[6]]), a Government Quarter named 'Breezy Point' at 2 Park Road (an original small crack was considerably widened[5]), and the then campus of St Joseph College at Robinson Road was severely damaged and had to be relocated (Figure 4). Furthermore, the occupants of a house with cracks at Peak Road had to be temporarily evacuated[7].

Figure 4

Figure 4. Memorial plaque in St Joseph College recording the then campus of the school at Robinson Road was severely damaged in the earthquake on 13 February 1918 and had to be relocated

Even though Hong Kong is not located within active seismic zone, it could still be affected by nearby earthquakes. The tremors in Hong Kong in the past several decades after the War were relatively minor with intensity V or below on the Modified Mercalli Scale. However, in view of the Nan'ao earthquake one hundred years ago bringing building damages and panic to the public, we should not be complacent but to learn from history and get prepared for future tremors.

Mok Hing-yim and Shun Chi-ming

Table 1. Earth tremors felt in Hong Kong on 13 and 14 February 1918[3]
Table 1

[1] Modified Mercalli Scale -
[2] Clergyman Hugh Anderson Moran worked in China and the Siberia from 1909 to 1918. The earthquake records during the period show that only the Nan'ao earthquake could cause such a scale of casualty shown in the photos
[3] Report of the Director of the Hong Kong Observatory for 1918 ( - the Rossi-Forel intensity scale, which is still being used at some places such as the Philippines (, was used in the report. The felt earth tremor at 2:07 pm on 13 February 1918 was classified as No.6 on the Rossi-Forel scale of 1 to 10, which approximately corresponded to intensity VI to VII on the Modified Mercalli Scale. The intensities for other felt earth tremors in Table 1, which are also shown on the Modified Mercalli Scale, were estimated from the intensity described in text in the report of the Director of the Hong Kong Observatory for 1918.
[4] The China Mail on 14 February 1918
[5] Hong Kong Telegraph on 13 February 1918
[6] Hong Kong Daily Press on 14 February 1918
[7] South China Morning Post on 14 February 1918